When Gavin was six, his younger brother Conor was killed by a car while they were walking to school. He received secondary education at Templeogue College, which he recalls the subjects as being "horrifying," with the exception of art classes, and studied horticulture at the College of Amenity Horticulture in Glasnevin, Ireland.
In May 2011 Gavin's "Irish Sky Garden" won the gold medal at the Chelsea Flower Show. However, it courted controversy when Cork City Council in Ireland agreed to put the garden on permanent display at Fitzgerald's Park in the Mardyke area of the city at a cost of at least €300,000 to the cash-strapped council. Already more than €1.7 million had been given to the project by Fáilte Ireland, the Irish tourism body. The decision attracted widespread criticism including that of a number of Cork city councillors. Councillor Ted Tynan pointed out that there were over 500 boarded up local council houses in the city and demanded clarification from the council as to what the exact cost of the garden would be. He is not participating in the 2017 Chelsea Flower Show.
Gavin once presented Surprise Gardeners, a Central TV series and a number of BBC television programmes including Gardeners' World; Home Front with Laurence Llewelyn-Bowen and, in 2004, a Twofour and RHS production Gardens Through Time. RTÉ is currently screening the television show I Want a Garden.
Gavin, presented an episode of Great British Garden Revival which aired on BBC Two in 2014.
In Autumn 2004, Gavin participated in BBC One's Strictly Come Dancing. In Summer 2006, he participated in Only Fools on Horses.
In 2010, Gavin took part in the first series of the ITV reality competition 71 Degrees North and was the third celebrity to become eliminated on 21 September 2010.
On 12 December 2010, Gavin appeared on Channel 4 on a Christmas edition of Come Dine with Me with David Gest, Hannah Waterman and Sherrie Hewson.
In January 2013, Gavin was a participant in the first series of Splash!. Gavin was the fourth celebrity to be eliminated from the show, despite low scores on his first night.
On 8 October 2013, Gavin appeared as a guest on The Sarah Millican Television Programme on BBC.
On 9 October 2013, Gavin appeared on the ITV game show Big Star's Little Star with his daughter Eppie.
On 14 December 2013, Gavin appeared on The Chase: Celebrity Special.
On 5 March 2014, he appeared in an episode of Sport Relief's Top Dog with his dog, Roxie.
Gavin won an RDS Gold Award in 1991 and 1993, a bronze medal at the Chelsea Flower Show in 1995, a silver-gilt medal in 2004, and a silver-gilt medal in 2007. He finally won a gold at Chelsea for "The Irish Sky Garden" in 2011, in partnership with Fáilte Ireland, and the City of Cork. He is also a patron at Southport Flower Show.
In 2007, Gavin received an Honorary Degree of Doctor of Art from Nottingham Trent University in recognition of his international reputation for contemporary garden designs.
Gavin is the author of a number of best-selling gardening books including Diarmuid Gavin's Big Ideas and Outer Spaces.
Gavin is married to Justine Keane, daughter of The Hon. Ronan Keane (the former Chief Justice of Ireland) and Terry Keane, and they have a daughter named Eppie born December 2004.